A step-doped porous type lithium manganese iron phosphate composite material and a preparation method thereof

Abstract

The application discloses a step-by-step doped porous lithium iron manganese phosphate composite material and a preparation method thereof. 2+ The application discloses a step-by-step doped porous lithium iron manganese phosphate composite material and a preparation method thereof. 2+ The application discloses a step-by-step doped porous lithium iron manganese phosphate composite material and a preparation method thereof. 2+ The application discloses a step-by-step doped porous lithium iron manganese phosphate composite material and a preparation method thereof. 2+ The application discloses a step-by-step doped porous lithium iron manganese phosphate composite material and a preparation method thereof.

Description

Technical Field

[0001] This invention belongs to the field of battery cathode material technology, and in particular relates to a stepwise doped porous lithium manganese iron phosphate composite material and its preparation method. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Lithium manganese iron phosphate (LiMn) x Fe 1-x PO4 (LMFP) is widely used in the cathode of lithium-ion batteries due to its advantages such as low cost, high safety, and long cycle life. With the rapid growth in electric vehicle sales, the problem of slow charging has become increasingly prominent, making it imperative to improve the charging speed of lithium-ion batteries.

[0004] Currently, commercially available lithium manganese iron phosphate (LMFP) offers higher energy density and higher operating voltage than lithium iron phosphate (LiFePO4, LFP), but it also suffers from drawbacks such as low electronic conductivity and low lithium-ion diffusion rate. LMFP has a conductivity of only 10⁻⁶. - 13 S·cm -1 The lithium-ion diffusion rate is 10. -15 cm 2 ·S -1 These figures are 1 / 10 and 1 / 10000 of those of the LFP, respectively. Compared to the 0.3 eV transition bandgap of the LFP, the electron transition bandgap in the LMFP is as high as 2 eV, which is essentially an insulator. This results in low electronic conductivity and ion mobility, thus limiting the charge and discharge rate of the LMFP.

[0005] The industry typically uses a one-step doping process to prepare a reaction precursor before the reaction, or a one-step direct doping method to prepare lithium manganese iron phosphate, thereby modifying lithium manganese iron phosphate and improving its rate performance. However, such methods cannot improve the structural stability of lithium manganese iron phosphate materials while improving rate performance. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a stepwise doped porous lithium manganese iron phosphate composite material and its preparation method. This preparation method can effectively improve the conductivity of lithium manganese iron phosphate and reduce the migration resistance of lithium ions, thereby enhancing the high-rate performance and stability of lithium manganese iron phosphate.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a stepwise doped porous lithium manganese iron phosphate composite material, comprising the following steps:

[0009] Ferrous salt, manganese salt, and high-valence metal ion salt were dissolved separately in phosphoric acid solution and stirred to obtain mixture 1; the valence of the metal ions in the high-valence metal ion source was greater than +3; the molar number of the high-valence metal ions was related to Mn 2+ and Fe 2+ The ratio of the total number of moles is 3-20:100;

[0010] While keeping the mixture 1 at 30-60℃ (keeping it at 30-60℃ to provide the temperature conditions for the reaction), ammonia water is added dropwise to the mixture 1 to co-precipitate;

[0011] The precipitate was washed, dried, and dehydrated, then ground with a lithium source, organic acid, and divalent metal ion source in water. After drying, it was calcined in an inert atmosphere to obtain the final product.

[0012] Mn 2+ Fe 2+ The ratio of the total number of moles of high-valence metal ions to the number of moles of divalent metal ions and lithium ions is 98.5-99.5:0.5-1.5:100.

[0013] In this invention, lithium manganese iron phosphate precursor, iron manganese phosphate, is prepared using high-valence transition metal oxide salts containing titanium, vanadium, and other elements as dopants. The iron manganese phosphate prepared in this step has certain lattice defects, and the introduction of high-valence metal elements gives the lithium manganese phosphate precursor the properties of a P-type semiconductor, thereby improving the electronic conductivity of the material.

[0014] Adding divalent metals such as Mg and Ca during lithium compounding can improve both crystal structure stability and lithium-ion mobility. Stepwise doping involves first constructing a stable P-type semiconductor precursor to improve the material's electronic conductivity, and then improving crystal structure stability. One-step doping can affect the uniformity of the P-type semiconductor, thus impacting the material's electronic conductivity.

[0015] In the lithium formulation process, organic acids, such as citric acid, are added to neutralize excess hydroxide ions and provide the organic carbon source required for carbon coating. The carbon coating layer formed by the organic carbon source can improve the rate performance and structural stability of lithium manganese iron phosphate while also suppressing Mn oxidation. 2+ The problem of dissolution.

[0016] In some embodiments, Mn 2+ and Fe 2+ The molar ratio is 7-9:2.

[0017] Preferred, Mn 2+ and Fe 2+ The molar ratio is 8:2.

[0018] In some embodiments, the high-valence metal ion source is ammonium metavanadate, metavanadic acid, titanium dioxide, titanic acid, or metatitanic acid.

[0019] In some embodiments, the divalent metal ion source is magnesium hydroxide, magnesium phosphate, magnesium carbonate, magnesium oxalate, calcium hydroxide, calcium phosphate, calcium carbonate, or calcium oxalate.

[0020] In some embodiments, the organic acid is citric acid, tannic acid, tartaric acid, malic acid, or ascorbic acid.

[0021] In some embodiments, the amount of organic acid added is 1.5%-2% of the total mass of the precursor and lithium carbonate.

[0022] Preferably, the amount of organic acid added is 1.5% of the total mass of the precursor and lithium carbonate.

[0023] In some embodiments, ammonia is added dropwise to adjust the pH of the solution to 5-7. This pH value is a reasonable value for co-precipitation; exceeding this range may cause some metal ions to precipitate prematurely or fail to precipitate at all.

[0024] In some embodiments, the calcination temperature is 650-750°C and the calcination time is 6-10 hours.

[0025] In some embodiments, the grinding is sand milling, and the sand milling time is 2-4 hours.

[0026] Preferably, the solid content in the slurry after sand milling is 25-35%.

[0027] In some embodiments, the drying is spray drying.

[0028] Secondly, the present invention provides a distributed doped porous lithium manganese iron phosphate composite material, which is prepared by the aforementioned preparation method.

[0029] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0030] The solution used in this invention is an acidic system prepared with phosphoric acid and deionized water, which provides phosphate groups while maintaining an acidic environment, thereby ensuring the dissolution of iron and manganese sources in the solution.

[0031] A small amount of Ti is added during the preparation of ferromanganese phosphate, the precursor of lithium manganese phosphate. 4+ and V 5+ High-valence metal ions, which will cause some Mn 2+ and Fe 2+ When Ti / V is replaced, it will occupy some of the Mn / Fe sites. 4+ and V 5+ The outermost d orbitals are unoccupied, Mn 2+and Fe 2+ The outermost d orbitals still contain some electrons. After Ti / V occupies some Mn / Fe sites, the precursor manganese iron phosphate exhibits an electron-deficient state, thus displaying the properties of a p-type semiconductor.

[0032] Mg was then added during the lithium preparation process. 2+ and Ca 2+ Divalent metal ions, in the subsequent reaction process, Mg 2+ and Ca 2+ It will replace part of Mn 2+ and Fe 2+ Mg 2+ and Ca 2+ The ionic radius is smaller than that of Mn 2+ and Fe 2+ The ionic radius of lithium iron phosphate (LFP) is such that after doping with Mg and Ca, the bond length of Mg / Ca-O becomes shorter than that of Mn / Fe-O, leading to a longer Li-O bond length after doping compared to before doping. This increased Li-O bond length effectively widens the Li migration channel, reducing the migration resistance of lithium ions and thus enhancing the rate performance of LFP. The added Mg... 2+ and Ca 2+ Ions do not undergo valence state changes during charging and discharging, thus playing a role in supporting structural stability.

[0033] In the lithium preparation process, organic acids, such as citric acid, are added to neutralize excess hydroxide ions and provide the organic carbon source required for carbon coating.

[0034] The doping of high-valence elements such as Ti / V creates defects in the crystal structure of lithium manganese iron phosphate and exhibits the properties of a P-type semiconductor. Carbon coating improves conductivity and increases the specific surface area of ​​the material. The combined effect of these two factors creates the porous structure of lithium manganese iron phosphate in this invention. Attached Figure Description

[0035] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0036] Figure 1 This is a schematic diagram of the structure of the distributed doped porous lithium manganese iron phosphate composite material prepared in the embodiments of the present invention.

[0037] In the figure, 1-Lithium manganese iron phosphate organic carbon layer; 2-Lithium manganese iron phosphate; 3-Pores created by elemental doping. Detailed Implementation

[0038] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0039] The present invention will be further described below with reference to the embodiments.

[0040] Example 1

[0041] A method for preparing a stepwise doped porous lithium manganese iron phosphate composite material includes the following steps:

[0042] 1. Add 85% phosphoric acid to ethylene glycol to prepare a phosphoric acid concentration of 0.75 mol / L.

[0043] 2. Prepare a 0.75 mol / L salt solution by taking MnSO4 and FeSO4·7H2O according to n(Mn):n(Fe)=8:2 and add it to solution 1 (the volume ratio of salt solution to solution 1 is 1:1). Mix and stir in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 2.

[0044] 3. Take Mn 2+ and Fe 2+ 5% of the total molar amount of ammonium metavanadate was added to solution 2 and mixed in a planetary mixer at 1000 rpm for 30 min to obtain solution 3.

[0045] 4. While controlling solution 3 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When precipitation has completely stopped, filter and collect the precipitate.

[0046] 5. The precipitate obtained in step 4 is washed, dried, and dehydrated to obtain lithium manganese iron phosphate precursor;

[0047] 6. Take magnesium hydroxide, citric acid (accounting for 1.5% of the total mass of precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+V):n(Mg):n(Li)=99.5:0.5:100, mix them with the precursor obtained in step 5, add water to prepare a slurry with a solid content of 30%, sand mill for 4 hours and spray dry.

[0048] 7. The particles obtained in step 6 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0049] Example 2

[0050] A method for preparing a stepwise doped porous lithium manganese iron phosphate composite material includes the following steps:

[0051] 1. Add 85% phosphoric acid to ethylene glycol to prepare a phosphoric acid concentration of 0.75 mol / L.

[0052] 2. Prepare a 0.75 mol / L salt solution by taking MnSO4 and FeSO4·7H2O according to n(Mn):n(Fe)=8:2 and add it to solution 1 (the volume ratio of salt solution to solution 1 is 1:1). Mix and stir in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 2.

[0053] 3. Take Mn 2+ and Fe 2+ 10% of the total amount of ammonium metavanadate was added to solution 2 and mixed in a planetary mixer at 1000 rpm for 30 min to obtain solution 3.

[0054] 4. While controlling solution 3 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When precipitation has completely stopped, filter and collect the precipitate.

[0055] 5. The precipitate obtained in step 4 is washed, dried, and dehydrated to obtain lithium manganese iron phosphate precursor;

[0056] 6. Take magnesium hydroxide, citric acid (citric acid accounts for 1.5% of the mass of the precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+V):n(Mg):n(Li)=99.5:0.5:100 and mix them with the precursor obtained in step 5. Add water to prepare a slurry with a solid content of 30%, sand mill for 4 hours and spray dry.

[0057] 7. The particles obtained in step 6 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0058] Example 3

[0059] A method for preparing a stepwise doped porous lithium manganese iron phosphate composite material includes the following steps:

[0060] 1. Add 85% phosphoric acid to ethylene glycol to prepare a phosphoric acid concentration of 0.75 mol / L.

[0061] 2. Prepare a 0.75 mol / L salt solution by taking MnSO4 and FeSO4·7H2O according to n(Mn):n(Fe)=8:2 and add it to solution 1 (the volume ratio of salt solution to solution 1 is 1:1). Mix and stir in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 2.

[0062] 3. Take Mn2+ and Fe 2+ 15% of ammonium metavanadate was added to solution 2 and mixed in a planetary mixer at 1000 rpm for 30 min to obtain solution 3.

[0063] 4. While controlling solution 3 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When precipitation has completely stopped, filter and collect the precipitate.

[0064] 5. The precipitate obtained in step 4 is washed, dried, and dehydrated to obtain lithium manganese iron phosphate precursor;

[0065] 6. Take magnesium hydroxide, citric acid (citric acid accounts for 1.5% of the total mass of the precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+V):n(Mg):n(Li)=99.5:0.5:100 and mix them with the precursor obtained in step 5 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, spray dry.

[0066] 7. The particles obtained in step 6 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0067] Example 4

[0068] A method for preparing a stepwise doped porous lithium manganese iron phosphate composite material includes the following steps:

[0069] 1. Add 85% phosphoric acid to ethylene glycol to prepare a phosphoric acid concentration of 0.75 mol / L.

[0070] 2. Prepare a 0.75 mol / L salt solution by taking MnSO4 and FeSO4·7H2O according to n(Mn):n(Fe)=8:2 and add it to solution 1 (the volume ratio of salt solution to solution 1 is 1:1). Mix and stir in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 2.

[0071] 3. Take Mn 2+ and Fe 2+ 10% of the total molar amount of ammonium metavanadate was added to solution 2 and mixed in a planetary mixer at 1000 rpm for 30 min to obtain solution 3.

[0072] 4. While controlling solution 3 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When precipitation has completely stopped, filter and collect the precipitate.

[0073] 5. The precipitate obtained in step 4 is washed, dried, and dehydrated to obtain lithium manganese iron phosphate precursor;

[0074] 6. Take magnesium hydroxide, citric acid (citric acid accounts for 1.5% of the total mass of the precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+V):n(Mg):n(Li)=99:1:100 and mix them with the precursor obtained in step 5 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, spray dry.

[0075] 7. The particles obtained in step 6 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0076] Example 5

[0077] A method for preparing a stepwise doped porous lithium manganese iron phosphate composite material includes the following steps:

[0078] 1. Add 85% phosphoric acid to ethylene glycol to prepare a reaction solution with a phosphoric acid concentration of 0.75 mol / L;

[0079] 2. Prepare a 0.75 mol / L salt solution by taking MnSO4 and FeSO4·7H2O according to n(Mn):n(Fe)=8:2 and add it to solution 1 (the volume ratio of salt solution to solution 1 is 1:1). Mix and stir in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 2.

[0080] 3. Take Mn 2+ and Fe 2+ 10% of the total molar amount of ammonium metavanadate was added to solution 2 and mixed in a planetary mixer at 1000 rpm for 30 min to obtain solution 3.

[0081] 4. While controlling solution 3 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When precipitation has completely stopped, filter and collect the precipitate.

[0082] 5. The precipitate obtained in step 4 is washed, dried, and dehydrated to obtain lithium manganese iron phosphate precursor;

[0083] 6. Take magnesium hydroxide, citric acid (citric acid accounts for 1.5% of the total mass of the precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+V):n(Mg):n(Li)=98.5:1.5:100 and mix them with the precursor obtained in step 5 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, spray dry.

[0084] 7. The particles obtained in step 6 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0085] Example 6

[0086] A method for preparing a stepwise doped porous lithium manganese iron phosphate composite material includes the following steps:

[0087] 1. Add 85% phosphoric acid to ethylene glycol to prepare a phosphoric acid concentration of 0.75 mol / L.

[0088] 2. Prepare a 0.75 mol / L salt solution by taking MnSO4 and FeSO4·7H2O according to n(Mn):n(Fe)=8:2 and add it to solution 1 (the volume ratio of salt solution to solution 1 is 1:1). Mix and stir in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 2.

[0089] 3. Take Mn 2+ and Fe 2+ A total of 10% tetramethyl titanium was added to solution 2 and mixed in a planetary mixer at 1000 rpm for 30 min to obtain solution 3.

[0090] 4. While controlling solution 3 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When precipitation has completely stopped, filter and collect the precipitate.

[0091] 5. The precipitate obtained in step 4 is washed, dried, and dehydrated to obtain lithium manganese iron phosphate precursor;

[0092] 6. Take magnesium hydroxide, citric acid (1.5% of the mass of precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+Ti):n(Mg):n(Li)=99:1:100 and mix them with the precursor obtained in step 5 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, spray dry.

[0093] 7. The particles obtained in step 6 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0094] Example 7

[0095] 1. Add 85% phosphoric acid to ethylene glycol to prepare a phosphoric acid concentration of 0.75 mol / L.

[0096] 2. Prepare a 0.75 mol / L salt solution by taking MnSO4 and FeSO4·7H2O according to n(Mn):n(Fe)=8:2 and add it to solution 1 (the volume ratio of salt solution to solution 1 is 1:1). Mix and stir in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 2.

[0097] 3. Take Mn 2+ and Fe 2+ 10% of the total molar amount of ammonium metavanadate was added to solution 2 and mixed in a planetary mixer at 1000 rpm for 30 min to obtain solution 3.

[0098] 4. While controlling solution 3 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When precipitation has completely stopped, filter and collect the precipitate.

[0099] 5. The precipitate obtained in step 4 is washed, dried, and dehydrated to obtain lithium manganese iron phosphate precursor;

[0100] 6. Take calcium hydroxide, citric acid (1.5% of the mass of precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+V):n(Ca):n(Li)=99:1:100 and mix them with the precursor obtained in step 5 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, spray dry.

[0101] 7. The particles obtained in step 6 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0102] Comparative Example 1

[0103] Steps 1 and 2 are the same as in Example 4;

[0104] 3. Take Mn 2+ and Fe 2+ Ammonium metavanadate with a molar ratio of 10% was added to solution 2, and magnesium hydroxide was added at a ratio of n(Mn, Fe, V):n(Mg) = 99:1. The mixture was stirred in a planetary mixer at a speed of 1000 rpm for 30 min to obtain solution 3.

[0105] Steps 4 and 5 are the same as in Example 4;

[0106] 6. Take lithium carbonate and citric acid (citric acid accounts for 1.5% of the total mass of the precursor and lithium carbonate) according to n(Mn, Fe, V):n(Mg):n(Li)=99:1:100 and mix them with the precursor obtained in step 5 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, spray dry.

[0107] Step 7 is the same as in Example 4.

[0108] Comparative Example 2

[0109] Steps 1 and 2 are the same as in Example 4;

[0110] 3. Under the condition of controlling solution 2 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When the precipitation has completely stopped, filter and collect the precipitate.

[0111] 4. The precipitate obtained in step 3 is washed, dried, and dehydrated to obtain the lithium manganese iron phosphate precursor;

[0112] 5. Take Mn 2+ and Fe 2+ Ammonium metavanadate with a total molar content of 10% was mixed with magnesium hydroxide, citric acid (citric acid accounts for 1.5% of the total mass of the precursor and lithium carbonate) and lithium carbonate according to n(Mn+Fe+V):n(Mg):n(Li)=99:1:100, and mixed with the precursor obtained in step 4 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, it was spray dried.

[0113] 6. The particles obtained in step 5 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0114] Comparative Example 3

[0115] Steps 1 and 2 are the same as in Example 4;

[0116] 3. Under the condition of controlling solution 2 at 50℃, gradually add ammonia water to solution 3 to adjust the pH of the solution to 5-7. When the precipitation has completely stopped, filter and collect the precipitate.

[0117] 4. The precipitate obtained in step 3 is washed, dried, and dehydrated to obtain the lithium manganese iron phosphate precursor;

[0118] 5. Take Mn 2+ and Fe 2+ Ammonium metavanadate with a total molar content of 10% was mixed with magnesium hydroxide and lithium carbonate according to the ratio n(Mn+Fe+V):n(Mg):n(Li)=99:1:100, and then mixed with the precursor obtained in step 4 to prepare a slurry with a solid content of 30%. After sand milling for 4 hours, it was spray dried.

[0119] 6. The particles obtained in step 5 are kept at 700°C for 10 hours under an inert gas atmosphere, and then crushed to obtain the final solid product.

[0120] The discharge capacities of the products prepared in Examples 1-7 and Comparative Examples 1-3 at different rates are shown in Table 1. As can be seen from the data in Table 1, Example 4, which employs a stepwise doping method—adding high-valence metal ions during the preparation of the lithium manganese iron phosphate precursor and small-radius divalent metal ions during lithium addition—and is supplemented with surface coating using an organic carbon source, exhibits a significant improvement in discharge capacity compared to the comparative examples at different rates such as 0.1C, 1C, 2C, and 5C.

[0121] Table 1. Discharge capacity (mAh / g) of the examples and comparative examples at different discharge rates.

[0122]

[0123] The Mn content in the electrolyte of the products prepared in Examples 1-7 and Comparative Examples 1-3 after 200 cycles at 5C is shown in Table 2. As can be seen from the data in Table 2, the examples that adopted a stepwise doping method of adding high-valence metal ions when preparing lithium manganese iron phosphate precursor and adding small-radius divalent metal ions when adding lithium, and supplemented by surface coating with organic carbon source, showed a significant reduction in Mn content in the electrolyte after 200 cycles at 5C compared with the comparative examples.

[0124] Table 2 shows the Mn content in the electrolyte of the examples and comparative examples after 200 cycles at a 5C rate.

[0125] Group Mn content in electrolyte (ppm) Example 1 399.8 Example 2 350.5 Example 3 402.7 Example 4 348.3 Example 5 349.6 Example 6 348.4 Example 7 348.7 Comparative Example 1 450.7 Comparative Example 2 480.2 Comparative Example 3 487.5

[0126] The electrode resistances of the products prepared in Examples 1-4 and Comparative Examples 1-2 are shown in Table 3. Table 3 shows that the electrode prepared under the same conditions (electrode thickness approximately 125 μm after double-sided coating and rolling) using a stepwise doping method—adding high-valence metal ions during the preparation of the lithium manganese iron phosphate precursor and small-radius divalent metal ions during lithium addition, supplemented by surface coating with an organic carbon source—showed a significant reduction in resistance compared to the comparative examples. This indicates that the stepwise doping method effectively improved the conductivity of lithium manganese iron phosphate, increasing its electrical conductivity.

[0127] Table 3 Electrode Resistors of Examples and Comparative Examples

[0128] Group Electrode resistance (Ω) Example 1 6.9 Example 2 6.5 Example 3 7.5 Example 4 6.1 Example 5 6.3 Example 6 6.2 Example 7 6.2 Comparative Example 1 8.0 Comparative Example 2 8.7 Comparative Example 3 9.8

[0129] The data above show that the porous lithium manganese iron phosphate composite material prepared by the stepwise doping method reduces the diffusion resistance of lithium ions, improves the structural stability (determined by the content of Mn ions in the electrolyte; the lower the content, the more stable the structure), and inhibits Mn dissolution. At the same time, the structure of the P-type semiconductor and the coating of the organic carbon layer improve the conductivity, thereby improving the high-rate performance and structural stability of the lithium manganese iron phosphate composite material.

[0130] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a stepwise doped porous lithium manganese iron phosphate composite material, characterized in that: Includes the following steps: Ferrous salt, manganese salt, and a high-valence metal ion source were dissolved separately in phosphoric acid solution and stirred to obtain mixture 1; the high-valence metal ion source had a valence greater than +3; the molar number of the high-valence metal ion was related to Mn 2+ and Fe 2+ The ratio of the total number of moles is 3-20:100; While keeping the mixture 1 at 30-60℃, ammonia water is added dropwise to the mixture 1 to carry out co-precipitation; The precipitate was ground with a lithium source, citric acid and a divalent metal ion source with water, then dried and calcined in an inert atmosphere to obtain the final product. Mn 2+ Fe 2+ The ratio of the total number of moles of high-valence metal ions to the number of moles of divalent metal ions and lithium ions is 98.5-99.5:0.5-1.5:

100. Mn 2+ and Fe 2+ The molar ratio is 8:2; The lithium manganese iron phosphate composite material obtained by the preparation method has a porous structure; The high-valence metal ion source is ammonium metavanadate, metavanadate, titanium dioxide, titanate, or metatitanic acid; The divalent metal ion source is magnesium hydroxide, magnesium phosphate, magnesium carbonate, magnesium oxalate, calcium hydroxide, calcium phosphate, calcium carbonate, or calcium oxalate. The calcination temperature is 650-750℃, and the calcination time is 6-10h.

2. The method for preparing the stepwise doped porous lithium manganese iron phosphate composite material according to claim 1, characterized in that: The amount of organic acid added is 1.5%-2% of the total mass of the precursor and lithium carbonate.

3. The method for preparing the stepwise doped porous lithium manganese iron phosphate composite material according to claim 1, characterized in that: Add ammonia water dropwise to adjust the pH of the solution to 5-7.

4. The method for preparing the stepwise doped porous lithium manganese iron phosphate composite material according to claim 1, characterized in that: The grinding process is sand milling, and the sand milling time is 2-4 hours; The solid content of the slurry after sand milling is 25-35%.

5. The method for preparing the stepwise doped porous lithium manganese iron phosphate composite material according to claim 1, characterized in that: The drying process is spray drying.

6. A stepwise doped porous lithium manganese iron phosphate composite material, characterized in that: It is prepared by any one of the preparation methods described in claims 1-5.

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